390 FOUNDATIONS OF BOTANY 



mixed with seeds of European weeds, will account for the 

 presence of many of the latter among us. Others have 

 been brought over in the ballast of vessels. Once landed, 

 European weeds have succeeded in establishing themselves 

 in so many cases, because they were superior in vitality 

 and in their power of reproduction to our native plants. 

 This may not improbably be due to the fact that the Euro- 

 pean and western Asiatic vegetation, much of it consisting 

 from very early times of plants growing in comparatively 

 treeless plains, has for ages been habituated to flourish in 

 cultivated ground and to contend with the crops which 

 are tilled there. 



458, Plant Life maintained under Difficulties. Plants 

 usually have to encounter many obstacles even to their 

 bare existence. For every plant which succeeds in reach- 

 ing maturity and producing a crop of spores or of seeds 

 there are hundreds or thousands of failures, as it is easy 

 to show by calculation. The morning-glory (Ipomoea pur- 

 purea) is only a moderately prolific plant, producing, in 

 an ordinary soil, somewhat more than three thousand 

 seeds. 1 If all these seeds were planted and grew, there 

 would be three thousand plants the second summer, sprung 

 from the single parent plant. Suppose each of these 

 plants to bear as the parent did, and so on. Then there 

 would be : 



9,000,000 plants the third year. 

 27,000,000,000 plants the fourth year. 

 81,000,000,000,000 plants the fifth year. 

 243,000,000,000,000,000 plants the sixth year. 

 729,000,000,000,000,000,000 plants the seventh year. 



1 Rather more than three thousand two hundred by actual count and 

 estimation. 



